This invention relates to the structure of a semiconductor device and the method of manufacture thereof and more particularly to the structure and fabrication method of a semiconductor device suitable for large-scale integrated circuits.
LSI circuits use semiconductor devices of various structures.
For instance, as disclosed in the Japanese Patent Application Laid-Open No. 255049/1986, bipolar transistors and MOSFETs (metal-oxide semi-conductor field-effect transistors) are formed on one and the same substrate to provide a BiCMOS LSI structure. The basic structure of the BiCMOS LSI is shown in FIG. 1, as applied to dynamic random access memory. In the figure, the bipolar transistor (simply referred to as a bipolar hereafter) forming the BiCMOS LSI is a vertical type and thus requires an n.sup.+ buried layer (in the npn bipolar). To form the n.sup.+ buried layer an epitaxial layer is grown.
In the above-mentioned conventional integrated circuits, forming the bipolar devices and the MOSFET devices requires an epitaxial process, which means reduction in the manufacturing time is difficult.
The conventional LSI circuits have no considerations as to sharing the manufacturing processes between the bipolar devices and the MOSFET devices.
Moreover, they incorporate no effective measures to increase the circuit density of LSI containing a combination of bipolar and MOSFET devices.